Process of making hardboard

ABSTRACT

A process for making cellulose hardboard in which a wet lap of cellulose fiber containing in excess of 25 percent water is subjected to a hot press at an initial high pressure step of 200 to 500 psi. The said cellulose fibers are then subjected to at least one intermediate pressure step of 60-150 psi to reduce the moisture content to less than 20 percent and finally subjecting the said cellulose fibers to a final pressure step of 5-20 psi to reduce the moisture content greater than 5 percent.

O Unlted States Patent [191 [111 3,793,139

Caresio Feb. 19, 1974 l PROCESS OF MAKING HARDBOARD [75] Inventor: John C. Caresio, Evergreen, Ill. f' 'G Leon Bfishore Assistant ExammerPeter Chm [73] As g Masonite corporatwn, g Attorney; Agent, or FirmMason, Kolehmainen, Rath- 22 Filed: Apr. 14, 1971 bum [21] Appl. No.: 134,016 I ABSTRACT i Apphcatlon Data A process for making cellulose hardboard in which a [63] $g g z 'g of July wet lap of cellulose fiber containing in excess of 25 a an one percent water is subjected to a hot press at an initial high pressure step of 200 to 500 psi. The said cellulose 5%] fibers are then subjected to at least one intermediate 58] .ld 1612 225 pressure Step of 60450 psi to reduce the moisture 1e 0 care content to less than 20 percent and finally subjecting the said cellulose fibers to a final pressure step of 5-20 [56] References cued psi to reduce the moisture content greater than 5 per- UNITED STATES PATENTS cent 2,046,750 7/1936 Mason 162/225 3,367,828 211968 Carter 162/225 5 Claims 2 Drawmg Flgll'es Pmmmre w 3.793.139

F|G.| PRESS PRESSURES AND TIME IOSEC. 362 PSI STEAM IN PRESS PLATENS 380-440psiq PRESSURE RSJ.

FIG 2 PRESS PRESSURES AND TIME IO SEC 362 PSI STEAM IN PRESS PLATENS 380-440psig PRESSURE P. $.l.

INVENTOR. JOHN C. CARESIO ATT'YS PROCESS OF MAKING I-IARDBOARD CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 843,126 filed July 18, 1969 now abandoned.

The present invention relates to a process of making hardboard, or hard fiber products, and is an improvement on the processes disclosed in Mason et al, U. S. Pat. No. 2,046,750, July 7, 1936 and Carter et al, U. S. Pat. No. 3,367,828, Feb. 6, 1968.

The Mason et al, patent discloses a pressure inversion process wherein extra high pressure is initially applied for a short interval to a fiber body, made up of lignocellulose fibers of trees, grasses, canes, cornstalks and the like. The fiber body is in the form of a wet lap containing a considerable amount of water, as for example, more than 25 percent of water, and usually more than 50 percent water. The wet lap is introduced in a heated platen press with wire mesh between a face thereof, preferably the lower face, and a press platen, and subjected to an initially high pressure to provide inversion of the mixture, thereby to press out substantially the maximum quantity of expressible liquid and to secure extra high surface qualities in such skin-like portion of the board for the smooth surface.

Carter et al describe a wet-pressing technique for forming low-density hardboard (25-50 lbs. per cubic foot) containing high moisture content (20-40 percent) in which the wet mat is subjected to pressure of about 300 psi. for about 2 minutes followed by a low pressure of about 30 psi. for about 8 minutes. The product of this press cycle is difficult to handle because of its high moisture content and inherent mechanical weakness.

By the procedure described herein it is possible to produce hardboard containing only -10 percent moisture which can be handled and treated like ordinary hardboard immediately after leaving the press. The press cycle is no longer than that described by Carter et al, but the product is much improved in strength and other physical characteristics.

In the production of hardboard, the major cost is in the press cycle, which requires usually 20 to 25 minutes for each batch of hardboard.

Presses for forming the hardboard are expensive, and reduction of press time per board results in a substantial saving in equipment cost and labor.

Accordingly it is an object of the present invention to provide an improved method of making hardboard.

A further object of the present invention is to provide an improved method of making hardboard wherein the press time is reduced.

It is a further object of the present invention to provide an improved method of making hardboard to obtain a desired dryness, using higher cooked fiber to get better weathering and to get higher internal bond strength.

Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In accordance with these and other objects, there is provided an improved method of making hardboard of a wet lap of cellulose fiber. A small percentage of petrolatum or other waterproofness-enhancing agent may be incorporated if desired. Moreover extraneous binders or fillers may be incorporated if desired. The wet lap is in the form of a slurry, containing an excess of 25 percent water, and usually about 60 percent water. In accordance with the present process, the wet lap is introduced into a heated press and is first subjected to a relatively high pressure pressing for inversion, wherein the moisture content of the wet lap is reduced, heat is provided or pumped into the wet lap, and the bottom wire and platen are flushed. The wet mat is then subjected to a stepped reduction, in one or more steps, of one or more intermediate pressures, for caliper control, to drive out a desired amount of remaining moisture content, and to reduce the moisture content in the mat to less than 20 percent, and usually to about 10-15 percent. Finally the mat is subjected to a third pressing operation at low pressure to relieve steam pressure and other internal stresses of the formed board so that the press may be opened and the boards removed without flowing, delamination, blistering, or other destruction of the board.

It has been found that by the use of the intermediate pressing steps, wherein the moisture content is reduced to less than 20 percent, and may be in the range of 5 percent to 12 percent, the board may be handled upon removal from the press without damage thereto. On the other hand removal of a board having moisture content above about 20 percent requires delicate and careful handling of the mat to prevent breakage or falling apart of the board. Moreover in accordance with the described process, the press time per board may be substantially reduced effecting a considerable saving in manufacturing expense. After removal of the board from the press, it is normally subjected to a heat treating cycle.

It has been found that initially subjecting a mat to a pressure in the range of 200 psi. to 500 psi., thereafter subjecting the mat in one or more steps to intermediate pressures in the range of 60 psi. to 150 psi., and finally subjecting the mat to a pressure-relieving step in the range of 5 to 20 psi. produces satisfactory results, reducing the moisture content to a desirable range, and permits removal of the mat from the press without damage to the hardboard. In some operations, it is desirable to include a second intermediate step at a minimum pressure of 25 psi. and a maximum of 50 psi.

FIGS. 1 and 2 illustrate typical press pressures plotted against press time for a mat having a finished caliper of approximately 7/16 inch at a specific gravity approximately 0.90.

FIG. 1 represents a typical IO-minute press cycle wherein the pressure on a mat, having 60 percent water content, is brought up to approximately 362 psi., held at that pressure for a short interval of time, and gradually reduced to a first intermediate pressure of 100 psi.

1 The mixture is in accordance with the teaching in the above-mentioned patent to Mason et al. It will be noted that in this first pressing step, to seconds is required to get the press to the operating pressure; then the 362 psi. operating pressure is held for approximately 10 seconds;-and the press then requires 5 to 20 seconds to come down to the firstintermediate pressure. The intermediate pressure of psi. is held for a period of time, and then reduced to a second intermediate pressure of 25 psi., being held at that pressure for a short period of time and gradually reduced to the final pressure of 5 psi. Each of the intermediate steps occurs over a 2-minute time range. The final or stressrelieving step takes place at 5 psi. and is held for a period of approximately 4 minutes.

FIG. 2 illustrates a similar mat, wherein the first intermediate pressure is reduced to 67 psi. Under these conditions the first intermediate pressure is maintained for a 4-minute period.

Subsequent to removal from the press, the hardboard was heat-treated in both the above illustrations for 2 hours at approximately 375 F.

lt has been found that the cycle described above in connection with FIG. 1, operating for minutes, produces a board with a resultant average moisture content of 8.2 percent; the cycle illustrated in FIG. 2 over the 12-minute period produced a lesser average moisture content of 5.3 percent. Such a board can be handled in like manner as a dry board as it is removed from the press. Moreover the provision of two intermediate steps provided accurate caliper control so that the board is of a desired thickness. Thus it is possible to obtain a desired board thickness without the use of caliper stops in the platen. The reduced time in press of the board on removal from the press minimizes pollution, cutting down smoke and other volatiles.

A series of press loads of 7/16 inch exterior siding were made on a commercial press at 10- and 12-minute press cycles. The inversion step was at 300 psi. over a period of 1.75 to 2 minutes, followed by a 4-minute intermediate step at 60 psi., a 2.25-minute intermediate step at 25 psi. and a final 4-minute releasing step of 5 psi. The hardboard had a specific gravity of 0.90 (54 lbs. per cu. ft.) and good physical properties.

In addition to reducing the press time by the use of the intermediate drying pressures, it has been found that the hardboard made by the method described herein has a higher ultimate strength than similar boards produced by other known methods. The internal bond strength was found to be in the range of 107 to psi. tensile strength perpendicular to the surface of the board compared with 90 psi. in board produced by the present commercial cycle. Moreover the board had a very smooth, cockle-free surface.

What is claimed as new and desired to be secured by letters Patent of the United States is:

1. The process of making hardboard of a wet lap of cellulose fiber containing in excess of 25 percent water, said process comprising the steps of pressing said wet lap in a hot press at an initial pressure of 200 to 500 psi. to provide inversion of the material, thereby reducing the free moisture content, pressing the wet lap at at least one intermediate pressure of 60 to psi. to reduce further the moisture content to less than 20 percent and control the final caliper, and thereafter finish pressing the material at a lower pressure of 5 to 20 psi. to relieve internal stresses and to permit removal of the hardboard from the press with a moisture content greater than 5 percent without destruction of the board.

2. The process of making hardboard as set forth in claim 1 including the step of heat treating the board after removal from the press.

3. The process of claim 1 wherein the moisture content of the board after removal from the press is 5-12 percent.

4. The process of making hardboard as set forth in claim 1 wherein said intermediate pressing step comprises pressing of the wet lap through two pressure ranges to provide a two-step intermediate pressing operation so that the caliper of the resulting hardboard is controlled without using caliper stops in said press.

5. The process of claim 4 wherein one intermediate pressing step is from 60-150 psi. and the other intermediate pressing step is 25-50 psi. 

2. The process of making hardboard as set forth in claim 1 including the step of heat treating the board after removal from the press.
 3. The process of claim 1 wherein the moisture content of the board after removal from the press is 5-12 percent.
 4. The process of making hardboard as set forth in claim 1 wherein said intermediate pressing step comprises pressing of the wet lap through two pressure ranges to provide a two-step intermediate pressing operation so that the caliper of the resulting hardboard is controlled without using caliper stops in said press.
 5. The process of claim 4 wherein one intermediate pressing step is from 60-150 psi. and the other intermediate Pressing step is 25-50 psi. 